The amine functionality reacts relatively readily with the epoxide group to give well-cured resins, very often at room temperature. For example, aliphatic polyamines are widely used for curing epoxy resins. Aliphatic amines generally can react at room temperature with epoxy resin in bulk to give cured products in reasonable periods of time from 24 to 48 hours. The major disadvantages of aliphatic amines is that they tend to be volatile and have high irritation potential, which precludes their use in many applications for toxicological reasons. Furthermore, aliphatic amines are relatively small molecules, causing inconvenient mixing ratios relative to their use with resinous material.
Aromatic amines, while giving cured epoxy materials of good chemical and thermal resistance, do not cure conveniently at room temperature and are suspected of being highly toxic and possibly carcinogenic.
To use the advantages of the amine cure mechanism, and to overcome the disadvantages associated with aliphatic polyamines and aromatic amines, the aliphatic amine groups have been incorporated into larger molecules which decrease irritation potential and contribute some selective improvement in handling and performance properties of the product. For example, amido polyamines and fatty polyamines have been used as curing agents for epoxies. The amido polyamines are derived from the reaction of a monobasic carboxylic acid and an aliphatic amine. The acids normally employed are C.sub.16, C.sub.18, and C.sub.19 fatty acid types derived from fats and oils, or copolymers of castor oil and styrene. The resulting fatty amido polyamines are low-viscosity liquids which cure according to the aliphatic polyamine and amide mechanisms.
Fatty polyamines are derived from dimerized linoleic acid and ethylenediamine or diethylenetriamine.
Fatty amido polyamines and fatty polyamines exhibit substantially the same properties. For example, both fatty amido polyamines and fatty polyamines cure bulk epoxy resin at ambient temperature and convenient mixing ratios. In addition, both fatty amido polyamines and fatty polyamines exhibit reduced volatility and reduced skin irritation potential compared to aliphatic polyamines, and demonstrate good flexibility and impact strength in the cured product.
However, compared to aliphatic polyamine curing agents, the corresponding fatty amido polyamines and fatty polyamines exhibit several significant disadvantages, including poor heat strength, as exhibited by cracks resulting from casting at peak exotherms, incompatibility with many epoxy resins, requirement for catalysis of thin films for rapid ambient temperature cure, and, in general, inferior chemical and solvent resistance.
Thus, there is a need for an ambient temperature curing system for epoxy resins which:
(1) has a reasonable pot life;
(2) cures quickly to a tough film;
(3) has the advantages of aliphatic polyamines, but avoids the toxic hazards of the aliphatic polyamines and aromatic amines; and
(4) cures epoxy resins to have better heat resistance and chemical stability than epoxy resins cured with the fatty polyamines and fatty amido polyamines.